What size air conditioner do you need?

Air conditioner size by room area

iHeat's 2026 guide offers these bands for standard ceilings, and describes them as a guide only:

Not every chart agrees, and Glow Green puts 28 to 33 square metres at 12,000 BTU, about 3.5kW. Since 2.5kW, 3.5kW and 5kW are the common sizes on sale, you are usually choosing between two.

How do kW and BTU compare?

They measure the same thing. One kW is about 3,412 BTU an hour, which makes 9,000 BTU about 2.6kW and 12,000 BTU about 3.5kW. Be sure you are reading cooling output and not power consumption, which is a far smaller number on the same label.

What moves the answer up or down?

South or west-facing glass can add a size by itself, and a conservatory needs its own sums. A vaulted ceiling holds more air than the floor suggests. A room directly under the roof runs hotter than one a floor down. New, well-insulated homes keep heat in, while solid-walled older ones lose it in winter. People and equipment count as well, so two screens in a home office or a busy kitchen will push it up. Timing matters too: a bedroom used at night is a different job from an office used all day.

What happens if the unit is too big or too small?

Slightly over-sized is no disaster now that inverter units can slow right down. Grossly over-sized costs more to buy, and in mild weather it can chill the air without pulling out much moisture, leaving the room clammy. Under-sized is worse, because it will run flat out on the one day you most want it and may still not get there.

Does sizing change if you also want heating?

Yes, so look at the heating output too. Winter heat loss through walls is a separate sum from summer heat gain. Average highs near Harlow go from about 8°C in January to about 23°C in July, so one room can have two quite different needs. We work out both when we visit. Read more in can air conditioning heat your home.